Multiple beam gun



2 Sheets-Sheet l Filed May 26, 1955 WWI/W (gw OR NE Y .V/ a /M m w w n mi ,7 P 0 R M wm xm* XMS I swm r uhh .H u NM, 9mm WN Hl J HNNHWIwm I l? w,a U .Jvva Nw w QN U E l l IU QQ w Dec. 6, 1955 R. E. BENWAY MULTIPLEBEAM GUN 2 Sheets-Sheet 2 Filed May 26, 1953 V INI/ENTOR. Robe/ 5.Bem/My BYW! TTORNY( United States Patent O MULTIPLE BEAM GUN Robert E.Benway, Lancaster, Pa., assigner to Radio Corporation of America, acorporation of Delaware Application May 26, 1953, Serial No. 357,511

Claims. (Cl. 313-70) This invention is directed to electron dischargedevices of the cathode ray tube type and more specifically to cathoderay tubes used as viewing tubes for color television.

One type of cathode ray viewing tube for color television, disclosed inthe copending application of Hannah C. Moodey, Serial Number 295,225,tiled .Tune 24, 1952, utilizes three electron guns for forming threeparallel electron beams arranged symmetrically about the tube axis anddirected at a target electrode within the tube envelope. A `combinedconverging and focusing lens, formed by potential differences betweenthree electrodes, is used to converge the three electron beams to acommon point on the target.

The target electrode consists principally of a masking electrode formedof a thin metal sheet having a large number of small apertures formedtherethrough. The apertured masking electrode is mounted substantiallynormal to the tube axis and in the paths of the electron beams.Conventional scanning coils are used to scan the three beamssimultaneously over the apertured mask electrode in a rectangularraster. A glass target support plate is mounted close and parallel tothe side of the masking electrode opposite to the electron guns.

There is formed, on the glass support plate, groups of phosphor dotswith each group consisting of three dots spaced 120 from a common point,which in turn is aligned with one of the apertures through the maskingelectrode, so that there is one group of phosphor dots aligned with eachaperture of the masking electrode. The three phosphor dots of each groupare formed respectively of phosphors luminescing in different colors asred, green, and blue.

Since the three parallel beams of the color tube are converged to acommon point on the masking electrode, the electrons of each beam strikethe masking electrode from substantially three diierent directions. Thebeams are scanned simultaneously by conventional scanning coils over themasking electrode surface. The electrons from the three beams will passthrough the apertures of the masking electrode along paths which arerespectively extensions of the directions from which the beams strikethe masking electrode, and will strike separate phosphor dots of eachgroup. This arrangement results inV the electrons then of each beamstriking phosphor dots which luminesce with one color.

A tube of the type described above can be utilized with various types ofcolor systems in which the color sig nals are either formed andtransmitted simultaneously or in a sequential order and in which eachsignal may cornprise an elemental signal or a frame signal. The signalsare applied each independently to the three guns of the viewing tube sothat the three electron beams are independently modulated to provide thecolor response at the correct time interval.

In tubes of the type described, the three electron beams are directedalong paths toward the target electrode. The i ice electron paths arespaced from each other and positioned symmetrically about a common axis.The electron lens fields, through which the beams pass, have theirgreatest uniformity at their centers. Furthermore, the scanning coilsare coaxially mounted about the common axis of the electron beams sothat the beams will pass into the deflecting elds substantially at thecenter of the elds. The three beams are deflected simultaneously fromthe tube axis and at the time of greatest detlection Will pass throughfringing portions of the deflecting fields, which are less uniform thanthe central portions of the fields. If the beams are widely spaced fromeach other, the fringing portions of the dellecting fields will causedeection distortion due to the delecting fields acting differently oneach beam. However, it is desirable that the deection of the three beamstogether be identical or substantially uniform. Deeetion distortionresults in a misconvergence of the three beams at the target and thebeams strike the target at separated spots. Misconvergence of the beamson the target is in direct relationship to the beam spacing in the yoke.Y

Because of deection distortion which results in misconvergence of thebeams of the target, as described above, the converging lens forbringing the three beams to a point of convergence at the screen must bevaried in accordance with the amount of deflection of the three beams.That is, a varying voltage is applied between the electrodes forming theconverging lens to modulatel the convergence of the three beams inaccordance with the deilection of the beams, to provide beamconvergenceat all angles of beam deection. This modulating voltage isknown as the dynamic voltage and varies in value up to 3,000 volts intubes of the type described.

Convergence of the three beams of the target is also directly related tothe alignment of the gun parts forming the electron beams as well as thesymmetry of the beam pathsabout a common axis. If gun parts aremisaligned or are unsymmetrically positioned in the tube, the convergingand deecting fields acting simultaneously on the beams will havediierent actions on the beams such that there will be a lack ofconvergence of the beams at the target.

It is, therefore, an object of this invention to provide a color picturetube of improved design.

It is another object of this invention to provide a color picture tubehaving a minimum of deection distortion.

It is also an object of this invention to provide a color picture tubehaving a reduced dynamic convergence voltage requirement.

It is also an object of this invention to provide a color picture tubehaving improved gun alignment in order to reduce beam distortion and toimprove picture resolution. v

The invention is specically related to a color picture or viewing tubehaving a masking electrode at the target whereby electrons approachingthe target from one of several directions will strike phosphor portionsof the target luminescing with a single` color of light. To reduce thedetlection distortion as well as the dynamic convergence voltagerequirements of the tube, the electrodes forming the converging lens ofthe tube are designed to provide a minimum of beam distortion. Also thegun is assembled with an improved alignment technique so that theelectron beams can be spaced closer together to minimize deflectiondistortion as Well as beam spot distortion at the target.

Figure 1 is a sectional view of portions of a color picture tube inaccordance with the invention.

Figure 2 is a sectional view along'line 2-'2 of Figure 1.

Figure 3 is a schematic representation of a target portion of thepicture tube of Figure 1.

Figure 4 is a schematic representation of the shape and action of thefocusing and converging fields of a tube similar to `that shown inFigure l.

Figure l shows a color television picture tube consisting of anevacuated envelope having an `enlarged bulb or shell portion 12 fixed toa tubular neck portion 14. The tubular neck 14 and the bulb portion 12are axially aligned along a common axis 16. Mounted at one end of theneck portion is an electron gun means 18 for providing a plurality ofelectron beams 15, 17 and 19 along paths which are symmetrically spacedfrom the axis 16. Mounted substantially normal to the axis 16 of thetube Vand within the opposite end of the bulb portion 12 is a targetelectrode assembly 20 consisting of a thin metallic sheet 22 tightlystretched in a closely spaced parallel relationship to a glass supportsheet 24. Details of the target construction and mounting are not givenhere but may be of the type disclosed, for example, as U. S. Patent2,611,100 of R. D. Faulkner et al.

The electron gun 18 consists of a plurality of electron sourcesconsisting of cathode electrodes 26, each of which are formed of shorttubular members mounted axially parallel in a ceramic insulating spacer28, which is in turn fixed Within the short tubular control electrode 30coaxially mounted within the neck portion 14. The cathode electrodes 26are closed at the end facing the target and are coated with thermionicemitting material (not shown) for providing sources of electrons.

The control electrode 30 is closed at the end facing the target 20 by aplate member 32 having a plurality of apertures therethrough, with oneaperture aligned with each coated end of the cathode electrodes 26.Closely spaced along the axis 16 `from the grid plate 32 is a firstaccelerating electrode plate 34 mounted in a parallel plane to that ofplate 32. Accelerating plate 34 is also apertured, with one aperturealigned with each of the apertures in plate 32.

Spaced along the axis 16 is a second accelerating electrode 38consisting of a tubular member closed at both ends by apertured plates40 and 42, respectively. Again, each aperture 41 in plate 42 is alignedwith a corresponding aperture in plate 40 and with an aligned pair ofapertures in plates 32 and 34 to provide a straight line path for thepassage of electrons from the respective cathode electrodes 26 throughthe accelerating electrode 38.

Mounted within the tubular accelerating electrode 38 is a beam maskingdiaphragm or plate member 44, also having a plurality of apertures withone aperture aligned with each of the straight Vline paths from cathodes26 through thc tubular electrode 38.

Spaced along the axis 16 and mounted coaxially thereto is a thirdaccelerating electrode 46 consisting of a short tubular member enclosingthe straight line paths from the cathodes 26 to the screen 20. A fourthaccelerating electrode is formed as a conductive coating 48 on the innersurface of the tubular envelope portion 14 and also encloses all of thestraight line paths from the several cathode electrodes 26 to the screen20.

The operation of the electron gun 18 is that in which appropriatevoltages are applied to the several electrodes as indicated in Figure l.These voltage values are in no way limiting but represent typicalvoltages which have been used to operate tubes of this type. Within eachcathode Velectrode 26 is mounted a heater filament (not shown), which,during tube operation, raises the cathode emissive material to athermionic temperature.

The electrons emitted lfrom each cathode pass through the alignedapertures in plates 32 and 34. The configuration of the electrostaticfield formed between plates 32 and 34, at `operating potentials, causesVthe electrons from each cathode to form a beam having 'a minimum crosssectional area or crossover point near plate 34. From each crossoverpoint 33 the electrons pass respectively into the tubular electrode 38as diverging beams. The fringe portions of each beam are masked ofi orcollected by the corresponding apertures of plate 44. The apertures 41in plate 42 are sufficiently large that the central portions of eachbeam, passing through the respective aperture of plate 44, do not strikethe edges of these apertures in plate 42.

Due to the differences of potential between electrodes 38 and 46 thereis formed an electron lens field 50 therebetween and as schematicallyshown in Figure 4. One portion of the lens A.field 50 extends throughthe apertures 41 in accelerating electrode plate 42 to form separatefields 52, with one field V52 in the path of each electron beam. Thefields 52 have a curvature of a degree that changes the individualelectron beam from divergence to convergence. The converging action offields 52 focuses each beam to a small spot on the masking electrode 22of target 20. Fields 52 constitute the main orprincipal focusing fieldfor each electron beam, respectively.

Between the third accelerating electrode 46 and the acceleratingelectrode coating 48, there is also established `an electron lens field54 having somewhat the configuration schematically shown in Figure 4.All of the beams pass through the common field 54 and the nature of thefield is to provide convergence of each beam toward the field axis whichis common with axis 16 of the gun and tube. Field 54 is adjusted tobring the several beams to a common point of convergence on the axis 16and at the target masking screen 22. The common converging eld 54 alsoadds somewhat to the focusing of the electrons in each beam to a minimumspot on the masking electrode 22 of the target. The operation of the gun18, as described above is similar to that described in the above citedcopending application of Hannah C. Moodey.

On the surface of the glass support plate 24 of target 2l), there isformed a large number of phosphor dots 56 and as shown more specificallyin the enlarged sketch of Figure 3. The phosphor dots 56 are arranged ingroups of three, with the dots in each group spaced about a commoncenter point. Each dot in every group is formed of a differentfluorescing phosphor material and, as indicated in Figure 3, thephosphor materials will luminesce with red, green, or blue light,respectively, when struck by high energy electrons.

As shown in Figure 3, the screen structure is such that the alignment ofeach separate phosphor dot with its respective aperture 58 is along adirectional line X, Y, or Z, each of which forms a small angle with theline joining the center of the aperture 58, with the center of therespective group of phosphor dots. Electrons passing through eachaperture S8 of Figure 3, along each one of the directional lines X, Y,or Z will strike only one of the phosphor dots. For the purpose ofclarity the portions of the Z and Y beams that project through apertures58 and hit phosphor dots 56 are not shown. The three electron beams 15,17 and 19 upon converging to a common spot on the masking screen 22 willpass through whatever apertures 58 are covered by the beam spot. Thus,from Figure 3 it can be seen that all of the electrons from one beampassing through electrode 22 will only strike those phosphor spots whichfluoresce with the same color and the screen 22 will mask the electronsof each beam from all of the other phosphor spots tiuorescing with theother colors.

Means `are provided for scanning the three beams of Figures 1 and 3 overthe surface of the target 20. The deliecting means are shownschematically as a yoke structure 60 mounted on thetubular envelopeportion 14. The yoke 60 is of a conventional design and operation and isconstituted principally of two pairs of coils, with the coils of eachpair connected in series with each other to a source lof lsaw 4-toothcurrents 'for providing line and fr'am scansion of the electron beamsover the surface of target 20. .The operation of the tube described isalso disclosed in greater detail in the above cited copendingapplication of Hannah C. Moodey.

The defiecting fields of yoke 60 act simultaneously on the three beamsto scan the beams vover the target in a normal rectangular raster.Figure 1 schematically shows the paths of the beams during one point oftheir deflection. The problem of scanning three spaced beamssimultaneously is different than that in scanning a single beam from theaxis of the defiection yoke. When the beams are deflected by the fieldsof yoke 60, they are directed ofr' the axis 16 into the frnging portionsof the ldefiecting fields. Since the beams are spaced from each otherand the distance from one beam to the other is of a significant amount,the beams at the same moment pass through different non-uniform portionsof the deiiecting field and will thus be deflected by different amountsat the edges of the rectangular raster. Because of this non-uniformityof the detiecting fields in the fringe areas the three beams converge atthree different spots, which are separated at the edge portions of theraster. Furthermore, it has been found that the separation of the threespots is in direct relationship to the separation of the beams in theyoke 60. To counteract this defiection distortion or beam separation atthe target, and to bring the beams to common convergence at all pointsof the scanned raster, it has been found necessary to modulate thevoltage of electrode 46 in accordance with the deflection of the beams.This modulating or dynamic convergence voltage, which is required,amounts to a maximum of 3,000 volts in some tubes of the type described.Close spacing of the beams in the defiection yoke 60 would be anadvantage in lowering the dynamic convergence voltage requirements.

It would appear that to provide closer spacing of the beams along theaxis 16, it would be merely a question of making the gun parts 18smaller and of using smaller spacing between the aligned apertures ofthe gun which define the several beam paths. However, it has been foundthat the cup electrodes 36 limit the closeness to which the apertures inplate 34 can be positioned, for example. Furthermore, it has also beenfound that if the apertures 41 of plate 42 are too closely spaced, thefocusing fields 52 will interfere with each other and cause distortionof the spot of each electron beam at the target.

In addition to deflection distortion described above, any misalignmentor distortion of gun parts may prevent accurate convergence. To providebetter aperture alignment, these electrode plates 32, 34, 40, 44, and 42are each formed with an additional three alignment apertures 45 as shownin Figure 2, in plate 42, for example. The

' gun parts then can be assembled with a mandrel having three rods, eachrod being threaded through one aperture 45 in each of the electrodeplates, together with appropriate spacers as is well known in the art.The mandrel rods are rigidly supported at both ends in accurate blocksand can be held to prevent possible twisting of the rods While theelectrode portions of the gun are sealed into the glass mounting beads47. This method of assembling eliminates the use of the gun or beamapertures in each of the plates as alignment apertures and thus preventstheir distortion, during gun assembly.

By thus utilizing a plurality of plates 32, 34, 40 and 42, for example,which can be accurately aligned as described, it is not necessary todepend upon the tubular electrodes 30 and 38, for example, for aperturealignment. Furthermore, the beam apertures in each of these electrodeplates can be accurately punched so that the apertures are spacedaccurately about a common center as shown in Figure 2, for example. Withthe improved alignment procedure, then, the common center of each platecan be mounted on the common tube axis 16. Therefore, the electronsdirected through the aligned r greater curvature.

6 apertures will be formed into beams accurately spaced, symmetrically,about the gun axis.

Figure 4 schematically discloses in detail the relationship of theconverging and focusing fields 54 and 50, respectively, with the spacingof the electron beams from the common axis 16. Figure 4 is not identicalwith the structure of Figure 1 since the apertures 41 in plate 42 arenot symmetrical to axis 16. The upper half of Figure 4 actuallyrepresents a gun of different dimensions than the lower half. The primenumbers, however, indicate the relationship of parts to correspondingparts in Figure l.

lt has been found as described above that in using progressively smallerbeam spacing in the gun, spot distortion will be produced at the screen.It has been found, on the other hand, that if the spacing of apertures41' from axis 16 is too great as shown in the upper portion of Figure 4,the electron beam 15 penetrates into the fringe area of fields 50 and54. The hatched sections, schematically shown along the beam path in theupper portion of Figure 4, substantially represents the cross sectionalshape of the electron beam 15 at each correspending point. It can benoted that as beam 15 emerges from the corresponding plate aperture 41it is focused by the converging portion of field 52 to provide a smallerand smaller cross sectional area, which becomes a minimum at the planeof screen 62. Also beam 15 passing throughaperture 41 is first caused todiverge toward the Afringe area of field 50 because of the curved natureof the field in that area. Beam 15 passes through this fringe area andenters the converging portion of field 54. However, as shown in Figure4, the upper portion of beam 15 passes through a more converging portionof field 54 so that the edge 68 of the beam is directed along a beampath 66, for example. Thus, the beam cross Sectional area begins to loseits uniformity as the more deflected portion 68 of the beam is deflectedtoward the center of the beam. As the beam moves toward the target, beamportion 68 follows path 66 and the distortion increases to an extentthat instead of all portions of the beam striking the target at a smalluniform spot, the beam strikes the target in a larger distorted spot.

The lower portion of Figure 4 discloses what substantially happens withan electron beam 17 which is more closely spaced to axis 16. Beam 17passes through aperture 41 into a uniform portion of field 50 and is notfirst diverged away from the axis 16 into the fringe areas of theconverging field 54. Rather, beam 17 passes through the more centralportions of fields 50 and 54, which have interacted to formsubstantially uniform concentric potential surfaces to provide little orno distortion of the beam and also to provide a uniformconvergenceaction on the beam. As shown in Figure 4, beam 17 passes intotheconverging portion of field 54 in an area where all portions of the beamare essentially uniformly refracted.

In accordance with the invention then, the design of the electron gun 1Sis that in which the electron beams are not refracted by the fringe ordistorting portions of fields 50 and 54. The diameter, as well as, thelength of the short tubular electrode 46 greatly determines the shape ofthe fields 50 and 54. In Figure 4, it can be seen that as the length ofelectrode 46 becomes larger, field 54 is moved away from field 50 sothat there is less interaction between the fields and each field assumesa This then moves the fringe or distorting portion of these fieldscloser to axis 16. This has approximately the same effect as. if eachbeam were moved away from the axis, as for example, from the position ofbeam in Figure 4 to the position of beam 15 of Figure 4.

Furthermore, it can be seen that, if the length of electrode 46 isgradually reduced, fields 50 and 54 will coact to a greater extent toeliminate the curvature of field 54 and thus reduce the convergenceaction of the field on the electron beams. Thus, there is an optimumrelationship between the spacing of the beam from axis 16 to the lengthof electrode 46, whereby optimum beam convergence will be provided. intubes of the type described, it has been found that, with an electrode46 having a diameter of 3A" an optimum length of electrode 46 is in theorder of 0.400" with a beam spacing of 0.145 from axis 16. The beam maybe moved away from axis 16 a distance of'approximately .160 before thereis any indication of spot distortion. Also, the length of electrode 46may be 0.200" more or less than the above dimension without causing anysignificant spe-t distortion.

Thus, with a minimum length of electrode 46 equal to approximately 0.200and a beam spacing from the axis 16 of 0.145, the ratio of the length ofthe electrode 46 to the spacing of the beam from axis 16 is in the orderof 1.4. The value of this ratio can only be reducer by making the lengthof electrode 46 smaller or the spa: i-

of the beam from axis i6 larger. Both of these conditions, however,would result in spot distortion so that the ratio value, 1.4, can beconsidered as a lower limit for optimum beam focus at the target. Thevalue of this ratio may be increased by increasing the length ofelectrode 46 within the limits described or by decreasing the spacing ofthe beam from axis 16. However, the spacing of cach beam from axis i6 isnot only limited by physical conditions in the manufacture of the gunT18, but also from the fact that the focusing fields 52 will interactwith each other if the spacing becomes too small.

The inside diameter of electrode 46 is also a controlling factor in thecondition of electron gun design. lf the diameter of electrode 46 isdecreased, both iields 54 and 50 will assume a greater curvature with aresulting movement of the fringe areas toward the axis 16 and thusprovides an eect similar to that produced by lengthening electrode 46.Increasing the diameter of electrode 46 has the opposite effect andreduces the convergence action of field 54 ori the several beams, asfields S and 59 then interact over a larger area and the resultant fieldbecomes atter. ln a similar manner, then, the diameter of electrode 46is related to the beam-to-axis spacing. With the 3/4 diameter describedabove, it has been found that there is a minimum ratio in the order of4.7 between the electrode 46 diameter and the beam-to-axis spacing.Increasing the beam-to-axis spacing decreases the ratio value andresults in beam distortion since the beam is moved into the fringeportions of elds 50 and 54. Also decreasing the diameter has a similareect and also prevides beam spot distortion. However, decreasingbeamto-axis spacing as well as increasing the diameter of electrode 4.6are both in the direction for eliminating spot distortion since thefringe areas of fields 50 and 54 and the beam paths are moved away fromeach other. However. as described above, for a given length of electrode46, there is also a limit to `which the diameter of the electrode may beincreased since the convergence action of field 54 is changedconsiderably with the increase in the diameter of electrode 46. Thedimensions previously given in which the diameter of electrode 4.6 issubstantially 3A and the beam spacing is .145 are optimum values andtogether provide a ratio of 5.2, which is above the minimum value statedabove of 4.7.

The variations in electrode dimensions discussed above are dependentupon the same voltages being present on the electrodes in every case.However, varying the voltage of electrode i6 and keeping the voltages ofelectrodes 48 and 38 constant will cause a greater change in beamconvergence than in the beam focusing. The focusing fields 52 are almostentirely independent of voltage changes within a large range of changeof the convergence iield 54. For example, the focusing action of fields52 is affected very little by change in the voltage of electrode 46 butthe contiguration of field 54 is considerably altered.

The results of moving the electron beams closer to axis 16andeliminating fringe elds in the .beam path has resulted in a smallerspacing of the beams from the axis `in thedeflection yoke ,60, whereby,there is less deflection distortion and more accurate convergence ofthe three beams over the Aentiretarget. This then results in a smallerdynamic convergence requirement and, in tubes of the type described,,Such voltage `requirements have been reduced from 3,000 volts to thatin the order of 200 volts with consequent circuitsmplification.

The improvement of aperture alignment has resulted also in a moresymmetrical arrangement of the beams about axis 16 whereby they passthrough identical portions of the converging and detiecting fields.Control of theVielationship of the length and diameter of electrode 46to the -spacing'of the beam from the axis 16 has resulted in theelimination of fringe fields from the paths of the respective electronbeams. This in turn has eliminated spot distortion of the beams at thetarget.

While certain specific embodiments have been illustrated and described,it will be understood that various changes and Vmodifications may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

l. An electron ,discharge vdevice comprising, electron gunmeans forproviding a plurality of beams along respective paths in a commongeneral direction and symmetrically spaced from a common axis, a targetelectrode mounted transversely to said beam paths, electrode meansbetween said gun and target for providing an electron lens in the pathsof said beams for converging said beams to a common point at saidtarget, said electrode means including a pair of tubular electrodescoaxially mounted on said axis and enclosing said beam paths, the ratioof the diameter of said tubular electrode nearer said gun to the spacingof said beams from said axis having a minimum value of 4.7.

2. An electron discharge device comprising, electron gun means forproviding a plurality of beams along respective paths in a commongeneral direction and symmetrically spaced from a common axis, a targetelectrode mounted transversely to said beam paths, electrode meansbetween said gun and target for providing an electron lens in the pathsof said beams for converging said beams to a common pointat said target,said electrode means including apair of tubular electrodes enclosingsaid beam paths, said pair of electrodes coaxially mounted on and spacedalong said axis, the ratio of the length of the tubular electrode ofsaid pair nearer said gun to the spacing of said beams from said axishaving a minimum value of 1.4.

3. An electron discharge device comprising, electron gun means forproviding a plurality of beams along respective paths in a commongeneral direction and symmetricallyspacedfrom a common axis, a targetelectrode mounted transversely to said beam paths, electrode meansbetween .said .gun and target for providing an electron lens in thepaths of said beams for converging said beams to a common point at saidtarget, said electrode means including a pair of tubular electrodesenclosing said beam paths, said pair of electrodes coaxially mounted onand spaced along said axis, the ratio of the length of the tubularelectrode of said pair nearer said gun to the spacing of said beams fromsaid axis having a minimum value of 1.4 and the ratio of the diameter ofsaid nearer electrode to the spacing of said beams from said axis havinga minimum value of 4.7.

4. An electron discharge device comprising. electron gun means forproviding a plurality of beams along respective paths in a commongeneral direction and symmetrically spaced from a common axis, a targetelectrode mounted transversely to said beam paths, electrode meansbetween said gun and target for providing an electron lens in the pathsof said beams for converging said beams to a common point at saidtarget, said electron gun including a tubular electrode coaxiallymounted on said axis and having a wall portion closing the end of saidtubular electrode faciugsaid target, said wall portion having apluralityfof apertures therethrough with said apertures symmetricallyspaced from said axis and with one of said apertures in the path of eachof said beams, said converging lens electrode means including a pair oftubular electrodes coaxially mounted and spaced along said axis fromsaid electrode wall portion, the ratio of the length of the tubularelectrode of said pair nearer said tubular electrode wall portion to thespacing of said apertures in said electrode wall portion from said axishaving a minimum value of 1.4.

5. An electron discharge device comprising, electron gun means forproviding a plurality of beams along respective paths in a commongeneral direction and symmetrically spaced from a common axis, a targetelectrode mounted transversely to said beam paths, electrode meansbetween said gun and target for providing an electron lens in the pathsof said beams for converging said beams to a common point at saidtarget, said electron gun including a tubular electrode coaxiallymounted on said axis and having a wall portion closing the end of saidtubular electrode facing said target, said wall portion having aplurality of apertures therethrough with said apertures symmetricallyspaced from said axis, with one of said apertures in the path of each ofsaid beams, said converging lens electrode means including a pair oftubular electrodes coaxially mounted and spaced along said axis fromsaid electrode Wall portion, the ratio of the length of the tubularelectrode of said pair nearer said tubular electrode Wall portion to thespacing of said apertures in said electrode wall portion from said axishaving a minimum value of 1.4, and the ratio of the diameter of saidnear electrode to the spacing of said apertures from said axis having aminimum value of 4.7.

References Cited in the file of this patent UNITED STATES PATENTS2,165,028 Blumlein `uly 4, 1939 2,659,026 Epstein Nov. 10, 19532,661,436 Van Ormer Dec. 1, 1953 2,663,821 Law Dee. 22, 1953

